Flushby rigs are used, inter alia, for clearing blocked oil and gas wells and removing damaged well lining from conventional oil and gas wells, coal seam gas wells and unconventional shale gas wells. During operation of some flushby rigs, the flushby rig is backed up to a well, jack legs are extended to level the deck of the rig, outriggers are extended as a safeguard against rollover of the rig, the teleboom (telescopic boom) of the mast is extended to lift a sheave at the upper end of the teleboom to operating height and the main boom of the mast is rotated into position above the well, and a drawworks winch is operated to lower the drawworks line and travelling blocks down to the well.
The sheave must be positioned directly above the well to provide a vertical drop for the drawworks line from the sheave to the well. It is critical that the drawworks line pass directly (i.e. vertically) upwardly from the well to the sheave. If, for example, the rig is to remove damaged well lining, the lining must be drawn directly upwardly to avoid fracturing and fragmenting of the casing and well head.
In the event that the sheave is not correctly positioned when the rig is set up, the mast is retracted and stowed, the outriggers and jack legs are drawn in and the position of the rig is reset. Thus considerable time is lost in repositioning the rig.
A mast can be connected to a support on the rig, by a pivot or hinge. The support is fixed to the deck of the rig and the mast pivots on the support between a stowed condition and an erect condition. To fix the mast in the erect condition, the outer boom is bolted to the support and guy ropes are provided to stabilize the mast. With the exception of lift raise cylinders used to lift the mast, the mast is held in position by the bolts and guy ropes. Due to the tremendous forces that can be applied through the mast by out of balance rope loads, particularly where impulse loads are experienced, such as in the event of a sudden blockage or where rocks are encountered, the mast is subject to oscillations back and forth. These problems can lead to premature fatigue and failure of the pivot, mast and support that connects the mast to the body of the rig.